$OpenBSD: patch-include_SAX_helpers_AttributesImpl_hpp,v 1.1 2017/05/21 09:50:45 espie Exp $
Nope, you can't add numbers to strings.

Index: include/SAX/helpers/AttributesImpl.hpp
--- include/SAX/helpers/AttributesImpl.hpp.orig
+++ include/SAX/helpers/AttributesImpl.hpp
@@ -8,6 +8,7 @@
 #include <SAX/Attributes.hpp>
 #include <stdexcept>
 #include <deque>
+#include <sstream>
 
 namespace Arabica
 {
@@ -545,9 +546,9 @@ class AttributesImpl : public Attributes<string_type, 
   ////////////////////////////////////////////////////////////////////
   void badIndex(unsigned int index)
   {
-    // sort out
-    std::string msg =
-	    "Attempt to modify attribute at illegal index: " + index;
+    std::ostringstream f("Attempt to modify attribute at illegal index: ");
+	f << index;
+    std::string msg = f.str();
     throw std::out_of_range(msg);
   }
 
